As noted below, all of the Steam for Mac launch titles offer a free Mac version to those that already own them on Steam PC thanks to the "Steam Play" feature, "which allows customers to purchase a game once and play it on all Steam supported platforms."

For a full breakdown of the various games available, hit up Steam or look below.

[Update - 1:05pm CST] As expected, Steam for Mac is now publicly accessible, enabling the download and play of the various titles one may already own via Steam.

[Original] Though you still can't access the service itself unless you were part of the closed beta, Valve's digital distribution client Steam is now downloadable on Mac--thanks Joystiq--with the service and its various games due to go public shortly.
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All of the launch titles offer a free Mac version to those that already bought them on Steam PC (and vice versa) care of the "Steam Play" feature, "which allows customers to purchase a game once and play it on all Steam supported platforms."

However, Valve has previously cautioned that "Steam Play" may not be universally supported across the entire Steam catalog, noting the functionality would be available "for our games and any of our partners who are interested in using the feature."

Moving forward, Valve plans to update the Steam for Mac catalog with new game "collections" each Wednesday--each collection is meant "to highlight specific functionalitites of Steam on the Mac"--as well as release native Mac OS/X versions of shooters Left 4 Dead, Left 4 Dead 2, Counter-Strike, and the Half-Life series.

"Mac and Windows players will be part of the same multiplayer universe [on Steam], sharing servers, lobbies, and so forth," Valve's John Cook previously explained.

In addition, Valve plans to release its future games simultaneously across Steam PC, Steam Mac and Xbox 360, with Portal 2--due this fall--marking the first such release.